A so-called two component type heat-sensitive recording material using a color forming reaction of a colorless or slightly colored electron donating dye precursor and an electron accepting compound is disclosed in Japanese Patent Publication Nos. 14039/70 and 4160/68.
The two component color forming type heat-sensitive recording material is prepared by pulverizing a colorless or slightly colored electron donating dye precursor and an electron accepting compound into fine particles and mixing a binder and the like therewith so that the two heat reactive compounds are separated by the binder and the like, and coating the mixture onto a support. Recording is conducted by heating one or both of the two heat reactive compounds to melt and contact each other, whereupon the color forming reaction occurs. Such a two component color forming type heat-sensitive recording material has great advantages in that (1) primary coloration takes place and color development is unnecessary, (2) a paper quality is nearly the same as a paper used for conventional types of recording, (3) handling of the material is easy, (4) the color forming density of the resulting colored images is high and (5) heat-sensitive recording materials having various color hues can easily be obtained. Therefore, it has a great practical value and is most widely used as a heat-sensitive recording material.
The use of the heat-sensitive recording material has recently been further developed particularly in the field of facsimile transmission, recording meters and printers. With wide spread, use of recording materials in the field of facsimile transmission, the recording speed has increased significantly. In view of this tendency to increase facsimile recording speed, a strong demand has arisen for heat-sensitive recording materials which require only a short pulse, that is, have the ability to undergo color formation with a low energy input. In other words, improvements in heat reactivity of the recording materials has long been desired. For example, it is disclosed in Japanese Patent Application (OPI) No. 47693/82 (the term "OPI" used herein refers to published unexamined Japanese patent application) that recording sensitivities can be increased by adjusting the volume average particle size of a color former to 2 .mu.m or less. Various approaches for improvements have been made and are disclosed in Japanese Patent Application (OPI) Nos. 187393/83, 67083/84 and 54884/85. However, with the increase in a heat responsive property, fog densities on the white areas also tend to increase, thereby reducing commercial values of the recording materials. Many approaches have been made to remove the above defect, as disclosed in Japanese Patent Application (OPI) Nos. 169889/84, 205796/83 and 167297/84, wherein improvements as to fog formation on the white areas are either not completely effective, or where improvements as to fog formation are effective, there is also largely a decrease in heat responsive property.